GENERAL SKETCH 35 



twelfth inch objective), the mass of endoplasm is seen to consist of a 

 more or less definite matrix, and if the cells be properly fixed and 

 stained, a distinct structure is visible. This appears to be little more 

 than a definite meshwork, the meshes of which are sometimes minute, 

 compressed, and narrow, sometimes large and open. The substance 

 of the mesh proper appears to differ noticeably from that within its 

 spaces. The latter is fluid-like, and not infrequently contains gran- 

 ules of larger or smaller size ; the former, also a fluid, appears more 

 dense, and is made up of exceedingly minute granules (microsomes). 

 Differential stains show that the various granules differ not only in 

 size, but in chemical composition, and it has been determined that 

 some are food particles in process of assimilation, and that others 

 are waste matters. This protoplasmic structure, which Biitschli 

 ('92) compares with a foam structure {ScJiaumplasma\ is described 

 by him as consisting of small drops of a liquid alveolar substance 

 inclosed within the meshes of a continuous inter-alveolar substance, 

 also liquid, but of a different composition. Each alveolus may be 

 compared to a bubble in a foam structure; the air of the bubble 

 corresponding to the alveolar substance, the walls to the inter- 

 alveolar substance. 



While the endoplasm of all Protozoa is alveolar in structure, there 

 is considerable variation in density due to the relative sizes of the 

 alveoli and to the nature of the granules contained within them ( Fig. 

 10, A-D). They vary in size from minute vesicles in Sporozoa (C) to 

 large vacuoles in many Heliozoa, Radiolaria, and Infusoria. In some 

 cases, e.g. in the heliozoon Actinosph&rium (D), or the cystoflagellate 

 Noctiluca, the vacuoles are so large that the protoplasmic structure 

 appears parenchymatous like a plant-cell. The granules in the walls 

 of the alveoli are equally variable in size. In some cases they are 

 exceedingly minute, and correspond apparently to the fine elementary 

 granules which Altmann ('94) regarded as the basis of all protoplasm 

 {e.g. Amoeba, A) ; in others they are coarse and obviously of different 

 kinds {Pelomyxa). 



The various granules within the alveoli are sometimes inert and 

 functionless and often crystalline in form. 1 In other cases they may 

 have some function to play in the economy of the cell. Thus car- 

 bohydrates in the form of starch, sugar, or cellulose are generally 

 present and serve as a reserve store of food, or of building material 

 for the outer covering. Other granules which are invariably present 

 may be food particles in various stages of digestion, assimilation, and 

 excretion, or oil particles of various forms and sizes. 



With the exception of the Sporozoa, every class of Protozoa includes 



1 Cf. Chap. IX., p. 286. 



